Neutrino clustering in the Milky Way and beyond

TL;DR

This paper models the local density of relic neutrinos in the Solar System by considering the gravitational effects of the Milky Way, Andromeda, and Virgo cluster, revealing significant clustering depending on neutrino mass.

Contribution

It presents the first three-dimensional calculation of relic neutrino density near Earth, including external structures like the Virgo cluster, improving previous spherical models.

Findings

Neutrino density enhancement varies with mass, up to 500% for 300 meV.

Virgo cluster significantly influences local neutrino clustering.

Neutrino mass affects the degree of local density enhancement.

Abstract

The standard cosmological model predicts the existence of a Cosmic Neutrino Background, which has not yet been observed directly. Some experiments aiming at its detection are currently under development, despite the tiny kinetic energy of the cosmological relic neutrinos, which makes this task incredibly challenging. Since massive neutrinos are attracted by the gravitational potential of our Galaxy, they can cluster locally. Neutrinos should be more abundant at the Earth position than at an average point in the Universe. This fact may enhance the expected event rate in any future experiment. Past calculations of the local neutrino clustering factor only considered a spherical distribution of matter in the Milky Way and neglected the influence of other nearby objects like the Virgo cluster, although recent $N$-body simulations suggest that the latter may actually be important. In this paper, we adopt a back-tracking technique, well established in the calculation of cosmic rays fluxes, to perform the first three-dimensional calculation of the number density of relic neutrinos at the Solar System, taking into account not only the matter composition of the Milky Way, but also the contribution of the Andromeda galaxy and the Virgo cluster. The effect of Virgo is indeed found to be relevant and to depend non-trivially on the value of the neutrino mass. Our results show that the local neutrino density is enhanced by 0.53% for a neutrino mass of 10 meV, 12% for 50 meV, 50% for 100 meV or 500% for 300 meV.